Rubber goods such as tire treads often are made from elastomeric compositions that contain one or more reinforcing materials such as, for example, particulate carbon black and silica; see, e.g., The Vanderbilt Rubber Handbook, 13th ed. (1990), pp. 603-04.
Good traction and resistance to abrasion are primary considerations for tire treads; however, motor vehicle fuel efficiency concerns argue for a minimization in their rolling resistance, which correlates with a reduction in hysteresis and heat build-up during operation of the tire. (A reduction in hysteresis commonly is associated with a decrease in tan δ0 value at an elevated temperature, e.g., 50° or 60° C., while good wet traction performance commonly is considered to be associated with an increase in tan δ value at a low temperature, e.g., 0° C.)
Reduced hysteresis and traction are, to a great extent, competing considerations: treads made from compositions designed to provide good road traction usually exhibit increased rolling resistance and vice versa. Filler(s), polymer(s), and additives typically are chosen so as to provide an acceptable compromise or balance of these properties. Ensuring that reinforcing filler(s) are well dispersed throughout the elastomeric material(s) both enhances processability and acts to improve physical properties. Dispersion of fillers can be improved by increasing their interaction with the elastomer(s), which commonly results in reductions in hysteresis (see above). Examples of efforts of this type include high temperature mixing in the presence of selectively reactive promoters, surface oxidation of compounding materials, surface grafting, and chemically modifying the polymer, typically at a terminus thereof.
Various elastomeric materials often are used in the manufacture of vulcanizates such as, e.g., tire components. In addition to natural rubber, some of the most commonly employed include high-cis polybutadiene, often made by processes employing catalysts, and substantially random styrene/butadiene interpolymers, often made by processes employing anionic initiators. Functionalities that can be incorporated into high-cis polybutadiene often cannot be incorporated into anionically initiated styrene/butadiene interpolymers and vice versa.
Polymers made by anionic initiation techniques frequently are coupled prior to use. Coupling can be effected by reacting two or more live (carbanionic) polymer chains with a coupling agent at or near the end of polymerization. Coupling can improve processability of such polymers and, in at least some cases, provide improved end use performance characteristics; for example, tire treads incorporating some tin-coupled polymers exhibit improved wear and reduced rolling resistance than treads incorporating similar but uncoupled polymers.
Commonly employed coupling agents include tin or silicon tetrahalides, e.g., SnCl4, SiCl4, etc. Such compounds can react with up to four live polymer chains. Such perfect stoichiometry is desirable but rarely achieved, however, because some of the coupling agent molecules can end up reacting with less than a stoichiometric number of live polymer chains. Increasing the amount of coupling agent can result in each coupling agent molecule having a less-than-stoichiometric amount of live polymer chains attaching thereto, while decreasing the amount of coupling agent can result in some live polymer chains remaining uncoupled. Maximizing the amount of polymer chains attached to each coupling agent compound while minimizing the number of free (uncoupled) polymer chains is desirable.
Complicating the foregoing is the fact that coupling efficiency of many types of coupling agents is affected by the identity and amount of polar compound(s) used in the polymerization (for purposes of modifying the microstructure of the polymer and, accordingly, its performance properties). Coupling efficiencies in batch processes that include polar modifiers typically are on the order of 50-60%, with even lower efficiencies being seen in continuous processes. See U.S. Pat. No. 6,489,403 for one method proposed for overcoming or ameliorating this reduction in efficiency.
Coupling reactions eliminate the possibility for post-polymerization functionalization of living polymers. Thus, to provide such polymers with terminal functionality, so-called functional initiators often are employed; see, e.g., U.S. Pat. Nos. 6,020,430 and 6,558,805 for two efforts in this regard.